It's hard for parents to stand idly by when their baby has a sore mouth from teething. But using prescription medicines such as viscous lidocaine may harm rather than help, the U.S. Food and Drug Administration says.

The FDA has issued a warning against using the gel-like prescription lidocaine 2% solution to treat infants and children while they're teething. Viscous lidocaine is a local anesthetic in a kind of syrup, commonly prescribed to adults to treat pain from mouth or throat ulcers that can occur during chemotherapy. But it is dangerous and potentially fatal to use on teething children ages 6 months to 3 years, according to FDA's drug safety communications report.

Viscous lidocaine is not approved by the FDA to treat teething in infants and young children, and accidental ingestion of the drug has been shown to result in seizures, severe brain injury and heart problems in children.

Symptoms can also include jitteriness, confusion, vision problems, vomiting, falling asleep too easily and shaking. And the symptoms may increase the risk of choking or breathing in food, according to FDA's consumer health report.

The FDA is requiring a "Boxed Warning," its strongest, to be added to the drug label.

"Topical pain relievers and medications that are rubbed on the gums are not necessary or even useful because they wash out of the baby's mouth within minutes, and they can be harmful," says pediatrician and FDA pathologist Ethan Hausman.

The FDA found 22 cases of serious adverse reactions to oral viscous lidocaine in children in 2014. Of those, six resulted in death, three were life-threatening, 11 required hospitalization, and two required medical intervention without hospitalization.

The cases were often caused by parents giving additional doses beyond what was prescribed or applying the medicine incorrectly. Other times, the lidocaine was accidentally ingested or there was a prescribing error.

U.S. pharmacies continue to dispense lidocaine to children younger than 2. More than 36,000 children up to age 2 received prescriptions for lidocaine 2% oral viscous products from U.S. outpatient retail pharmacies in 2012, according to data collected from IMS Health, an information and technology services company for the health care industry. That accounted for about 4% of all patients who received dispensed prescriptions for lidocaine.

Two years ago, the FDA also recommended that parents not use benzocaine products to treat teething children without the supervision of a health care professional. Benzocaine is a local anesthetic and can be found in over-the-counter products such as Anbesol , Hurricaine, Orajel and Orabase. Benzocaine is potentially fatal to children and can lead to a reduction in oxygen in the bloodstream, according to the FDA's consumer report.

"Teething is a normal phenomenon; all babies teethe," Hausman said in the FDA consumer health report.

THE FDA'S TIPS TO TREAT TEETHING BABIES:

• Gently rub or massage the child's gum with your finger.

• Chill a teething ring or wet washcloth in the refrigerator for a short time. Make sure it's cool and not cold like an ice cube. If it's too cold, it can hurt the child's gums. The coolness soothes the gums by dulling the nerves.

• Supervise children so they don't accidentally choke on the teething ring or washcloth.